Presently, heavy CRT display has been gradually replaced by flat panel display. Popular flat panel display comprises liquid crystal display (LCD) and organic light emission display (OLED).
In the LCD, each pixel unit is driven by a corresponding thin film transistor (TFT) in a TFT array substrate and cooperates with external circuits so as to realize display of images. In the active matrix organic light emission display (AMOLED), each OLED pixel unit is driven by a corresponding TFT in a TFT array substrate and cooperates with external circuits so as to realize display of images. In the displays described above, the TFT is used as a switch element and is a key element for image display. The performance of the TFT directly determines the performance of the flat panel display.
The TFT which has been industrialized mainly comprises amorphous silicon TFT, polycrystalline silicon TFT, monocrystalline silicon TFT, and amorphous silicon TFT is most frequently used for the TFT array substrate of the flat panel display.
Recently, metal oxide TFT has been researched and developed. The metal oxide TFT has the advantage of high carrier mobility, in this case, the size of the TFT can become smaller, and thus the resolution of the flat panel display can be increased and the display quality can be improved. Furthermore, the metal oxide TFT further has advantages of uniform characteristics, low cost of materials and processes, low process temperature, suitable for coating process, high transmittance, large band gap and the like.
Conventionally, the TFT array substrate comprising the metal oxide TFT is manufactured by six photolithography processes. If the number of the photolithography processes is reduced, i.e. if the number of mask to be used is reduced, the production efficiency will be improved and the production cost will be decreased.